1. Field of the Invention
This invention relates to methods of producing clad metals and more particularly to methods of covering the surface of metals with dissimilar materials having desired properties such as good resistance to corrosion, high-temperature corrosion, oxidation and wear.
2. Description of the Prior Art
With the recent development in industries and technologies, many materials have come to be used in increasingly severe environments. For example, oil and natural gas produced today contain much hydrogen sulfide and carbon dioxide (such oil and gas are known as sour oil and sour gas). As low-alloy steels corrode an crack when exposed to sour oil and gas, such nickel-base alloys as Hastelloy C-276 and Inconel 625 have been used for oil-well and line pipes. (Hastelloy and Inconel are the trade names for groups of nickel-base corrosion-resistant alloys.) Because they are very expensive, clad metals covered with thin layers of such expensive alloys have come into use too, with the base metal (such as low-alloy steel) providing the strength required by individual applications.
Various processes for making seamless and welded cald-steel pipes and rolling clad-steel plates have been established and proposed. But all conventional processes are complex and low in yield. Clad steels with such nickel-base alloys as Hastelloy C-276 and Inconel 625, especially in tubular form, are so difficult to make that none has been put into commercial use. The inventors found that the alloys are much more resistant to the deformation induced by hot working than the base metal of low-alloy or carbon steel. Thus, the cladding alloy and base metal cannot be formed evenly by hot rolling or other ordinary processes. Dissimilar metals deforming independently defy the joining of pipes or other structural members.
Stellite (the tradename for a series of cobalt-base alloys) and other similar alloys are either overlaid or sprayed on to valve spindles, pistons and cylinders of reciprocating pumps and other sliding members and slurry transportation pipe requiring wear resistance. Ni-Cr, Ni-Cr-Al-Y, Co-Cr-Al-Y and other oxidation-resistant alloys are either overlaid or sprayed onto pressure vessels and steel pipes used in high-temperature environments. But overlaying or spraying dissimilar metals onto finished products extremely pushes up their production costs. Furthermore, it is impossible to apply them to such small spaces as the inside of small-diameter pipes.
Hot isostatic pressing is a well-known technology. Several clad metals utilizing this cladding technology have been proposed. For example, Japanese Provisional Patent Publication No. 223106 of 1986 discloses an efficient process for making high-alloy clad metals in which a powder of high-alloy metal is heated to above the solidus-line temperature thereof under pressure. But all of the conventionally reported or proposed hot isostatic pressing processes are costly because they are applied to finished products. Besides, they are not applicable to large or long (over 12 m in length, for example) products.
Japanese Provisional Patent Publications Nos. 190007 and 190008 of 1986 disclose two methods. In one of them a powder material is packed and sealed in a capsule composed of a malleable metal cylinder with a heavier wall thickness and a metal cylinder with a lighter wall thickness. The powder is the compressed into a billet by cold isostatic pressing. The obtained billet is hot extruded into a desired shape. In the other method, a cylindrical piece of a malleable metal is placed in a concentrically double-walled cylindrical container of rubber or other similar substance, whit the malleable metal held in contact with the inner wall of the container. A powder material is then packed and sealed between the outer wall of the container and the cylindrical malleable metal and compressed by cold isostatic pressing. The obtained billet is hot extruded into a desired shape. But neither of them solves the problems of the separation from the base metal and the cracking of the cladding layer subjected to hot extrusion of materials clad with such nickel-base alloys as Hastelloy C-276 and Inconel 625 and other similar materials highly resistant to hot working and inadequately adherent to the base metal.
In Japanese Patent Application No. 40644 of 1988 (or U.S. Pat. No. 4,844,863), the inventors proposed a method of cladding the surface or metal with a dissimilar metal by hot isostatic pressing in which a powder of the dissimilar metal is heated to a temperature not higher than the solidus-line temperature thereof under a pressure and shaping the obtained clad piece by hot working, a method of shaping the obtained clap piece by hot working after applying a solution treatment, and a method of shaping the obtained clad piece by hot working immediately after applying a soaking treatment.